Solving proteins at non-atomic resolution by direct methods: Update

Direct methods can be used to solve proteins of great structural complexity even when diffraction data are at non-atomic resolution. However, one of the main obstacles to the wider application of direct methods is that they reliably phase only a small fraction of the observed reflections, those with a sufficiently large value of the normalized structure factor amplitude. The subsequent phase expansion and refinement required for full structure solution are difficult. Here a new phase refinement procedure is described, which combines (1-2) difference Fourier synthesis with electron density modification techniques and the vive la difference and Free Lunch algorithms. This procedure is able to solve data resistant to other direct space refinement procedures.